The present invention relates to a support material for three-way catalysts containing platinum group metal, which catalysts are employed for the purification of exhaust gases of internal combustion engines. More especially, the support material of this invention is optionally doped with one or more oxides of non-noble metals. The catalysts for the purification of car exhaust gases produced in accordance with the invention have a reduced tendency for H.sub.2 S emission. In more particular detail, the present invention relates to support material consisting of active aluminum oxide and at least 10 wt.% of cerium oxide, relative to the sum of all support constituents.
Modern high-performance exhaust gas catalysts store considerable amounts of sulfur dioxide originating from the fuel in the form of sulfate and sulfite, during the lean mode operation of the engine. When enriching the fuel mixture, for example when accelerating the vehicle, the sulfur compounds thus stored by the catalyst are reduced by the hydrogen present in the exhaust gas and released rapidly into the atmosphere in the form of hydrogen sulfide. As hydrogen sulfide emission of this type is undesirable for reasons relating to environmental protection and health, there was an urgent need to suppress it.
European application 0 244 127 proposes the addition of nickel to catalysts in order to combat the problem. However, as nickel and its compounds are not completely harmless as far as health is concerned, the preparation and handling of catalysts containing nickel require costly safety measures. There was therefore a need to develop car exhaust gas catalysts which considerably suppress hydrogen sulfide emission even without the presence of nickel additives.
As high-performance catalysts for car exhaust gas purification currently have higher cerium oxide contents, and cerium oxide shows a strong tendency to store sulfur dioxide, the emission of hydrogen sulfide is predominantly influenced by the form in which the cerium oxide is present; namely, as a component of the catalyst support material or as the support material.
It has now been found that the problem outlined can be solved if at least the cerium oxide component of the support material, and preferably a predominant part of all cerium oxide present, is used in the form of spray dried particles.